Draft: backends/drm: add support for the "power saving policy" property

This property allows the compositor to hint to the driver that it should disable some power saving strategies, which can interfere for example with color accuracy

draft because this property isn't merged yet into the kernel (see https://lists.freedesktop.org/archives/dri-devel/2024-May/454838.html), and we should expose this in kscreen

Depends on libraries/plasma-wayland-protocols!79, and libkscreen!189 for changing the setting